Radio Frequency Identification (RFID) tags are used in a multiplicity of ways. They may be used in locating and identifying accompanying objects, as well as for transmitting information about the state of an object. It has been known since the early 60's that electronic components of transponders could be powered by a sequence of periodic signal bursts sent by a reader (or interrogator) and received by a tag antenna on each of the transponders.
The RF electromagnetic field induces an alternating current in the transponder antenna that can be rectified by a RF diode of the transponder. The rectified current can be used for a power supply for the electronic components of the transponder, and enables the transponder to broadcast a return signal without itself having a self-contained power supply.
An illustrative cycle of a prior art operation of an array of ten RFID tags may be described as follows:    1. The base station or the reader is on channel one and RFID tags 1–8 respond by beginning their participation in the identification protocol. All eight tags are successfully identified.    2. The reader now hops to channel 2, and the frequency of channel 2 allows tags 7–9 to be powered. Tag 9 will now respond by beginning participation in the identification protocol, while tags 1–6 lose their power and therefore stop participating. Since tags 7 and 8 were already identified and continue to be powered sufficiently when operating on channel, they do not participate in the protocol.    3. The reader hops to channel 3. The frequency of channel 3 allows tags 2–10 to be powered. Tags 7–9 stay powered and do not participate in the protocol. However, tags 2–6 must be reidentified in order to identify the one truly new tag 10.
The RFID tags that are not well powered lose track of state information. This state information is essentially a bookmark in the communication sequence between the RFID tag and the base station. In running an ID protocol, for example, tags that newly enter the field, as well as tags that have lost power and then regained it while remaining in the field, are treated equally (i.e. tags that have lost power and regained it may be identified a second time). This process of again identifying an RFID tag that has previously been identified is, of course, inefficient. U.S. Pat. No. 6,404,325 (Heinrich et. al.) is directed at maintaining the integrity of state information retained by a Radio Frequency Transponder during a loss of power. In Heinrich, the state information is maintained by a mirror latches mechanism and a capacitor utilized as a power source for the mirror latches mechanism during the interval when the power supply to the RFID tag is interrupted. The time interval during which the mirror latches mechanism retains the state information (persistence of state) depends on the size of the capacitor. Persistence of state is also determined by the leakage current from the capacitor.
Of course, leakage may be primarily due to switches connected to the capacitor and not through the capacitor itself. The switches are open, and so only semiconductor leakage current flows through them.
Because the leakage current may vary with temperature, an RFID tag with a certain size capacitor may not retain state information as long as necessary at high temperatures and retain state information longer than is practicable at low temperatures.